In the field of lighting technology, there are many different ways for an individual to provide light to a space, such as a room in a house. In the past, lighting levels were limited to only two positions, namely ON and OFF. Therefore, when a person wished to light up the space, they simply turned the light on. In the more recent past, in order to allow individuals to control the amount of lighting in a space, and to assist in lowering the cost to power the lighting, techniques were created which allowed the lighting to be dimmed so that the light was not always operating at a maximum level. These dimming techniques allow for lighting to also be colour-mixed so that different coloured lighting can be combined with each other to provide a plurality of different colours to light up the space.
The creation of light emitting diodes (LEDs) has also helped enhance the field lighting technology and has also assisted in lowering overall lighting costs. The combination of dimming techniques and LEDs has provided an improvement over existing lighting. Some existing techniques for dimming and/or colour mixing LEDs include Pulse Width Modulation, Variable Frequency Modulation, Bit Amplitude Modulation and Pulse Amplitude Modulation.
Pulse Width Modulation (PWM) is a method that uses a signal at a constant frequency with dimming achieved by varying the pulse width and therefore, the duty cycle of the pulse. However, when performing PWM in the digital domain (DPWM), problems occur when the system is operating at a low intensity.
If DPWM is used with 1024 steps, for example, the change in intensity from 1 step out of 1024 to 2 steps out of 1024 is a factor of two. The human eye is capable of detecting a change in intensity as little as 1% so this will be detected as a very large step change in intensity. A clock is used to determine when the pulse is to turn on and off. The clock counts at fixed intervals, and repeats at a multiple of that interval. For example, a 10-bit clock containing 1024 steps will go from 1021 to 1022 to 1023 and back to 0. The pulse on time duration begins when the clock resets to 0 and may end when the clock reaches some modulated value, for example “2” representing an intensity level. If a lower intensity level is required, the only option is to go to “1” which represents a 50% drop in intensity level and is easily seen as a large step.
Variable Frequency Modulation is a method which uses a signal having a constant pulse width. Dimming is achieved by varying the off time and therefore, frequency and duty cycle of the signal. However, this technique suffers from the problem that a wide dynamic frequency range is required for the dimming current to achieve an acceptable range of light intensities. For example, assuming a constant pulse width of 50 microseconds (in some topologies it takes a certain amount of time for current to ramp up and down into the LED since the LED cannot reach peak current instantly), a 1% light intensity value will have a dimming current frequency of 200 Hz (50/5000 microseconds).
In the case of a 50% LED light intensity, the dimming current frequency will be 10 Khz (50/100 microseconds). Electromagnetic compatibility (EMC) issues limit maximum high frequency for LEDs. Some LED manufacturers recommend maximum frequencies for LEDs in the order of 1 KHz since minimizing frequency variation is a desirable goal.
Bit Angle Modulation utilizes a binary pulse train that contains the light intensity value. Each bit of the pulse train is stretched proportionally to the binary significance of each bit and each bit of the binary word is therefore allocated a fixed range of phase angles within the drive cycle.
Another method is pulse amplitude modulation (PAM) in which a signal is converted to a digital signal and an analog channel through simple transformations, and vice versa.
It is also possible to vary the intensity of an LED by varying the amount of current passing through it but this can lead to a problems such as colour shifting.
It is, therefore, desirable to provide a novel method of modulation for the dimming and/or colour mixing of LEDs.